The human ITPA polymorphic variant P32T is destabilized by the unpacking of the hydrophobic core

Peter D. Simone, Lucas R. Struble, Admir Kellezi, Carrie A. Brown, Corinn E. Grabow, Irine Khutsishvili, Luis A Marky, Youri I Pavlov, Gloria E Borgstahl

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

Inosine triphosphate pyrophosphatase (ITPA), a key enzyme involved in maintaining the purity of cellular nucleoside triphosphate pools, specifically recognizes inosine triphosphate and xanthosine triphosphate (including the deoxyribose forms) and detoxifies them by catalyzing the hydrolysis of a phosphoanhydride bond, releasing pyrophosphate. This prevents their inappropriate use as substrates in enzymatic reactions utilizing (d)ATP or (d)GTP. A human genetic polymorphism leads to the substitution of Thr for Pro32 (P32T) and causes ITPA deficiency in erythrocytes, with heterozygotes having on average 22.5% residual activity, and homozygotes having undetectable activity. This polymorphism has been implicated in modulating patients' response to mercaptopurines and ribavirin. Human fibroblasts containing this variant have elevated genomic instability upon treatment with base analogs. We find that the wild-type and P32T forms are dimeric in solution and in the crystal structure. This abolishes the previous speculation that the P32T change disrupts dimerization as a mechanism of inactivation. The only difference in structure from the wild-type protein is that the area surrounding Thr32 is disrupted. Phe31 is flipped from the hydrophobic core out into the solvent, leaving a hole in the hydrophobic core of the protein which likely accounts for the reduced thermal stability of P32T ITPA and ultimately leads to its susceptibility to degradation in human cells. Circular dichroism and thermal denaturation studies confirm these structural results. We propose that the dimer of P32T variant subunit with wild-type subunit is degraded in cells similarly to the P32T homodimer explaining the level of loss of ITPA activity in heterozygotes.

Original languageEnglish (US)
Pages (from-to)197-208
Number of pages12
JournalJournal of Structural Biology
Volume182
Issue number3
DOIs
StatePublished - Jun 1 2013

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Inosine Triphosphate
Pyrophosphatases
Heterozygote
Hot Temperature
Deoxyribose
6-Mercaptopurine
Genomic Instability
Ribavirin
Medical Genetics
Homozygote
Dimerization
Genetic Polymorphisms
Circular Dichroism
Guanosine Triphosphate
Nucleosides
Proteins
Hydrolysis
Fibroblasts
Adenosine Triphosphate
Erythrocytes

Keywords

  • Genomic instability
  • Hydrophobic surfaces
  • Inosine triphosphate pyrophosphatase
  • Nucleotide pool
  • Protein stability
  • X-ray crystallography

ASJC Scopus subject areas

  • Structural Biology

Cite this

The human ITPA polymorphic variant P32T is destabilized by the unpacking of the hydrophobic core. / Simone, Peter D.; Struble, Lucas R.; Kellezi, Admir; Brown, Carrie A.; Grabow, Corinn E.; Khutsishvili, Irine; Marky, Luis A; Pavlov, Youri I; Borgstahl, Gloria E.

In: Journal of Structural Biology, Vol. 182, No. 3, 01.06.2013, p. 197-208.

Research output: Contribution to journalArticle

Simone, Peter D. ; Struble, Lucas R. ; Kellezi, Admir ; Brown, Carrie A. ; Grabow, Corinn E. ; Khutsishvili, Irine ; Marky, Luis A ; Pavlov, Youri I ; Borgstahl, Gloria E. / The human ITPA polymorphic variant P32T is destabilized by the unpacking of the hydrophobic core. In: Journal of Structural Biology. 2013 ; Vol. 182, No. 3. pp. 197-208.
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abstract = "Inosine triphosphate pyrophosphatase (ITPA), a key enzyme involved in maintaining the purity of cellular nucleoside triphosphate pools, specifically recognizes inosine triphosphate and xanthosine triphosphate (including the deoxyribose forms) and detoxifies them by catalyzing the hydrolysis of a phosphoanhydride bond, releasing pyrophosphate. This prevents their inappropriate use as substrates in enzymatic reactions utilizing (d)ATP or (d)GTP. A human genetic polymorphism leads to the substitution of Thr for Pro32 (P32T) and causes ITPA deficiency in erythrocytes, with heterozygotes having on average 22.5{\%} residual activity, and homozygotes having undetectable activity. This polymorphism has been implicated in modulating patients' response to mercaptopurines and ribavirin. Human fibroblasts containing this variant have elevated genomic instability upon treatment with base analogs. We find that the wild-type and P32T forms are dimeric in solution and in the crystal structure. This abolishes the previous speculation that the P32T change disrupts dimerization as a mechanism of inactivation. The only difference in structure from the wild-type protein is that the area surrounding Thr32 is disrupted. Phe31 is flipped from the hydrophobic core out into the solvent, leaving a hole in the hydrophobic core of the protein which likely accounts for the reduced thermal stability of P32T ITPA and ultimately leads to its susceptibility to degradation in human cells. Circular dichroism and thermal denaturation studies confirm these structural results. We propose that the dimer of P32T variant subunit with wild-type subunit is degraded in cells similarly to the P32T homodimer explaining the level of loss of ITPA activity in heterozygotes.",
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T1 - The human ITPA polymorphic variant P32T is destabilized by the unpacking of the hydrophobic core

AU - Simone, Peter D.

AU - Struble, Lucas R.

AU - Kellezi, Admir

AU - Brown, Carrie A.

AU - Grabow, Corinn E.

AU - Khutsishvili, Irine

AU - Marky, Luis A

AU - Pavlov, Youri I

AU - Borgstahl, Gloria E

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Y1 - 2013/6/1

N2 - Inosine triphosphate pyrophosphatase (ITPA), a key enzyme involved in maintaining the purity of cellular nucleoside triphosphate pools, specifically recognizes inosine triphosphate and xanthosine triphosphate (including the deoxyribose forms) and detoxifies them by catalyzing the hydrolysis of a phosphoanhydride bond, releasing pyrophosphate. This prevents their inappropriate use as substrates in enzymatic reactions utilizing (d)ATP or (d)GTP. A human genetic polymorphism leads to the substitution of Thr for Pro32 (P32T) and causes ITPA deficiency in erythrocytes, with heterozygotes having on average 22.5% residual activity, and homozygotes having undetectable activity. This polymorphism has been implicated in modulating patients' response to mercaptopurines and ribavirin. Human fibroblasts containing this variant have elevated genomic instability upon treatment with base analogs. We find that the wild-type and P32T forms are dimeric in solution and in the crystal structure. This abolishes the previous speculation that the P32T change disrupts dimerization as a mechanism of inactivation. The only difference in structure from the wild-type protein is that the area surrounding Thr32 is disrupted. Phe31 is flipped from the hydrophobic core out into the solvent, leaving a hole in the hydrophobic core of the protein which likely accounts for the reduced thermal stability of P32T ITPA and ultimately leads to its susceptibility to degradation in human cells. Circular dichroism and thermal denaturation studies confirm these structural results. We propose that the dimer of P32T variant subunit with wild-type subunit is degraded in cells similarly to the P32T homodimer explaining the level of loss of ITPA activity in heterozygotes.

AB - Inosine triphosphate pyrophosphatase (ITPA), a key enzyme involved in maintaining the purity of cellular nucleoside triphosphate pools, specifically recognizes inosine triphosphate and xanthosine triphosphate (including the deoxyribose forms) and detoxifies them by catalyzing the hydrolysis of a phosphoanhydride bond, releasing pyrophosphate. This prevents their inappropriate use as substrates in enzymatic reactions utilizing (d)ATP or (d)GTP. A human genetic polymorphism leads to the substitution of Thr for Pro32 (P32T) and causes ITPA deficiency in erythrocytes, with heterozygotes having on average 22.5% residual activity, and homozygotes having undetectable activity. This polymorphism has been implicated in modulating patients' response to mercaptopurines and ribavirin. Human fibroblasts containing this variant have elevated genomic instability upon treatment with base analogs. We find that the wild-type and P32T forms are dimeric in solution and in the crystal structure. This abolishes the previous speculation that the P32T change disrupts dimerization as a mechanism of inactivation. The only difference in structure from the wild-type protein is that the area surrounding Thr32 is disrupted. Phe31 is flipped from the hydrophobic core out into the solvent, leaving a hole in the hydrophobic core of the protein which likely accounts for the reduced thermal stability of P32T ITPA and ultimately leads to its susceptibility to degradation in human cells. Circular dichroism and thermal denaturation studies confirm these structural results. We propose that the dimer of P32T variant subunit with wild-type subunit is degraded in cells similarly to the P32T homodimer explaining the level of loss of ITPA activity in heterozygotes.

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KW - Hydrophobic surfaces

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KW - Nucleotide pool

KW - Protein stability

KW - X-ray crystallography

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